Ionization chambers are standard dosimetry detectors generally used in radiotherapy. An ionization chamber comprises a polarization electrode separated from a collector electrode by a gap or space comprising a fluid of any nature whatsoever (including air).
Several types of ionization chambers are encountered, for example such as so-called cylindrical ionization chambers and ionization chambers comprising parallel plates. Cylindrical ionization chambers comprise a central or axial electrode generally in the form of very fine cylinder, isolated from the second electrode in the form of a hollow cylinder or a hood surrounding said central or axial electrode. Ionization chambers comprising parallel plates have a collector electrode separated from a polarization electrode, said collector and polarization electrodes being planar and parallel to one another.
The fluid comprised in the gap or space separating the collector and polarization electrodes of an ionization chamber used in dosimetry is most often a gas, neutral or not. When an ionizing beam passes through the ionization chamber, there is an ionization of the gas comprised between the electrodes and ion-electron pairs are created. An electric field is generated by applying a potential difference between the two electrodes of the ionization chamber. The presence of the electric field makes it possible to separate these ion-electron pairs and cause them to drift on the electrodes, creating a current at said electrodes that will be detected.
The curve of FIG. 1 is one example of the evolution of the amplitude of the electrical pulse received by the collector electrode as a function of the electric potential difference between the collector electrode and the polarization electrode. This curve can be divided into six zones covering the different gas detector states:
Z1: unsaturated state;
Z2: saturated state;
Z3: proportional state;
Z4: limited proportionality state;
Z5: Geiger-Müller state;
Z6: continuous discharge state.
In zone Z1, called the unsaturated state zone, when the electric field between the two plates is nonexistent, there is a recombination of the ion-electron pairs. By applying an increasing electric potential difference between the two electrodes, the resulting electric field increasingly efficiently separates the ion-electron pairs, and the recombination phenomena are attenuated. The positive and negative charges are driven toward their respective electrodes more and more quickly, as a function of the intensity of the electric field, reducing the ion concentration equilibrium in the gas, and consequently, the number of recombinations. The current measured in the ionization chamber increases with the electric field created in the ionization chamber, reducing the lost charge quantities. When an electric field created between the two electrodes is powerful enough, the recombination effects become negligible and all of the charges created by the ionization process contribute to measuring the current. At that level, the charge collection efficiency is maximal and increasing the potential difference between the two electrodes will no longer make it possible to increase the measured current, since all of the created charges are already collected and their formation speed is constant. One is then in zone Z2, called the saturated state zone, where the dosimetry measurements in the ionization chambers are generally done in radiotherapy. Under these conditions, the measured current is a good indication of the dose deposited by a beam in the volume of the ionization chamber.
Several factors can harm the saturation of an ionization chamber. The most important of these is the recombination phenomenon. This phenomenon can be minimized by adjusting the different parameters of the ionization chamber, such as, for example, the thickness of the gap between the two electrodes, the nature and/or pressure of the gas comprised in that gap, etc. The recombination effects can also depend on the size and/or shape of the beam. The recombination phenomena will also increase proportionally as a function of the intensity of the current of the beam. The current loss percentage due to the recombinations and therefore the error percentage of the current that is measured below the real saturation region increases proportionally with the intensity of the current. For less intense beams, the recombination effect is less decisive. To measure high-intensity beams, a high enough potential difference between the electrodes is required to work under saturation conditions.
For the currents of very high-intensity beams, like those encountered in advanced radiotherapy techniques, the technological usage limit of traditional ionization chambers is reached. The recombination phenomena become very significant and then, a reliable measurement correction method is crucial.
It would be possible to work in the so-called unsaturated state zone Z1 close to the saturated state zone by taking into account the errors due to the recombination, which are significant. In that case, it is necessary to know the saturation levels of the ionization chamber as a function of the beam current. A calibration curve using the intensity of the beam current as a function of the intensity of the collected current can be done by measuring the ionization currents as a function of the beam current, with the aim of knowing the beam current. But for this calibration to remain valid, it is necessary for the other parameters, such as the potential difference applied between the two electrodes, the gap, the pressure inside the ionization chamber, the energy, the size and shape of the beam, to remain constant. Another flaw in this method is that it does not make it possible to differentiate between a variation of the signal due to a beam current and a variation of the signal due to a deregulation of one of the parameters of the ionization chamber. To offset these measurement problems, new dosimetry devices making it possible to measure beam currents in a wide intensity range are necessary.